The present disclosure relates generally to glass sheets and more specifically to a glass soot deposition and sintering process for forming uniform glass sheets such as silica glass sheets.
Glass sheet materials can be formed using a variety of different methods. In a float glass process, for example, a sheet of solid glass is made by floating molten glass on a bed of molten metal. This process can be used to form glass sheets having uniform thickness and very flat surfaces. However, float glass processes necessarily involve direct contact between the glass melt and the molten metal, which can lead to undesired contamination at the interface and less than pristine surface quality. In order to produce high quality float glass sheets with pristine surface properties on both major surfaces, float glass is typically subjected to surface polishing steps, which add additional expense. Moreover, it is believed that the float process has not been used to make rollable glass sheets.
An additional method for forming glass sheet materials is the fusion draw process. In this process, molten glass is fed into a trough called an “isopipe,” which is overfilled until the molten glass flows evenly over both sides. The molten glass then rejoins, or fuses, at the bottom of the trough where it is drawn to form a continuous sheet of flat glass. Because both major surfaces of the glass sheet do not directly contact any support material during the forming process, high surface quality in both major surfaces can be achieved.
Due to the dynamic nature of the fusion draw process, the number of glass compositions suitable for fusion draw processing is limited to those that possess the requisite properties in the molten phase (e.g., liquidus viscosity, strain point, etc.). Further, although relatively thin glass sheets can be made via fusion draw, the process cannot be used to form rollable high-silica glass sheets. Finally, the apparatus used in the fusion draw process can be expensive.
In addition to their limitations with respect to thin glass sheet materials, both float and fusion draw processes are largely impractical sheet-forming methods for high-silica glass sheets due to the high softening point (˜1600° C.) of silica. Rather, silica glass substrates are typically produced by cutting, grinding and polishing silica ingots produced in batch flame-hydrolysis furnaces. Such a batch approach is extremely expensive and wasteful. Indeed, the requisite slicing and polishing that would be needed to produce uniform, thin, flexible silica glass sheets via flame-hydrolysis would likely render the process prohibitively expensive.
In view of the foregoing, economical, uniform, thin, flexible, rollable glass sheets having a high surface quality are highly desirable. The glass sheets can comprise one or more layers, components, or phases. Such glass sheets can be used, for example, as photo mask substrates, LCD image mask substrates, and the like.
A method of forming glass sheets involves a glass soot deposition and sintering process. According to various embodiments, the deposition involves depositing glass soot particles on a deposition surface of a soot-receiving device to form a supported soot layer, removing the soot layer from the deposition surface to form a first soot sheet, and depositing additional glass soot particles on at least one surface of the first soot sheet to form a composite soot sheet. The composite soot sheet can be sintered to form a glass sheet. The glass sheet can be thin, rollable glass sheet, or a thicker glass sheet.
The supported soot layer, which is released from the deposition surface, has two major opposing surfaces. The “contact surface” is the surface that is formed in contact with the deposition surface of the soot receiving device, while the “free surface” is the opposing surface. In a further embodiment, after the supported soot layer is released from the deposition surface to form a first soot sheet, additional glass soot particles can be deposited on one or both of the “contact surface” or the “free surface.” Moreover, after the formation of at least two soot layers, still further glass soot particles can be deposited to form a multi-layer composite soot sheet. In the context of a composite soot sheet, an interfacial surface means a surface between one soot layer and a subsequently-deposited soot layer, while a free surface means a surface that has been formed without direct contact with another surface.
A high-silica glass sheet made using the foregoing approach can have an average thickness of 150 microns or less and an average surface roughness over at least one of two major opposing surfaces of 1 nm or less. By depositing a second soot layer on the contact surface of the first soot sheet, it is possible to form a composite soot sheet, and ultimately a sintered glass sheet, where neither of the exposed major surfaces of the glass sheet has been in contact with a deposition surface. Such pristine surfaces can result in a high-quality glass sheet.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.